Electrochemical method of producing nano-scaled graphene platelets

ABSTRACT

A method of producing nano-scaled graphene platelets with an average thickness smaller than 30 nm from a layered graphite material. The method comprises (a) forming a carboxylic acid-intercalated graphite compound by an electrochemical reaction; (b) exposing the intercalated graphite compound to a thermal shock to produce exfoliated graphite; and (c) subjecting the exfoliated graphite to a mechanical shearing treatment to produce the nano-scaled graphene platelets. Preferred carboxylic acids are formic acid and acetic acid. The exfoliation step in the instant invention does not involve the evolution of undesirable species, such as NO x  and SO x , which are common by-products of exfoliating conventional sulfuric or nitric acid-intercalated graphite compounds. The nano-scaled platelets are candidate reinforcement fillers for polymer nanocomposites. Nano-scaled graphene platelets are much lower-cost alternatives to carbon nano-tubes or carbon nano-fibers.

GOVERNMENT GRANT INFORMATION

Based on the research result of a US Department of Energy (DoE) SmallBusiness Innovation Research (SBIR) project. The US government hascertain rights on this invention.

This is a co-pending application of a US patent application submitted onJul. 21, 2007 by Aruna Zhamu, Joan Jang, Jinjun Shi, and Bor Z. Jang,entitled “Method of Producing Ultra-thin, Nano-Scaled Graphene Plates.”

FIELD OF THE INVENTION

The present invention relates to a method of producing nano-scaledgraphene platelets (NGPs) or graphite nano-platelets. The methodcomprises a step of electrochemically intercalating a layered graphitematerial, such as natural graphite, graphite oxide, and other laminargraphite compounds, to produce a graphite intercalation compound (GIC).This step is followed by exfoliation of the GIC and separation of theexfoliated graphite flakes to form NGPs, particularly NGPs with anaverage thickness of no more than 2 nm or 5 layers (i.e., 5 graphenesheets or 5 layers of basal plane).

BACKGROUND

Carbon is known to have four unique crystalline structures, includingdiamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube(CNT) refers to a tubular structure grown with a single wall ormulti-wall, which can be conceptually obtained by rolling up a graphenesheet or several graphene sheets to form a concentric hollow structure.A graphene sheet is composed of carbon atoms occupying a two-dimensionalhexagonal lattice. Carbon nano-tubes have a diameter on the order of afew nanometers to a few hundred nanometers. Carbon nano-tubes canfunction as either a conductor or a semiconductor, depending on therolled shape and the diameter of the tubes. Its longitudinal, hollowstructure imparts unique mechanical, electrical and chemical propertiesto the material. Carbon nano-tubes are believed to have great potentialfor use in field emission devices, hydrogen fuel storage, rechargeablebattery electrodes, and as composite reinforcements.

However, CNTs are extremely expensive due to the low yield and lowproduction rates commonly associated with all of the current CNTpreparation processes. The high material costs have significantlyhindered the widespread application of CNTs. Rather than trying todiscover much lower-cost processes for nano-tubes, we have workeddiligently to develop alternative nano-scaled carbon materials thatexhibit comparable properties, but can be produced in larger quantitiesand at much lower costs. This development work has led to the discoveryof processes for producing individual nano-scaled graphite planes(individual graphene sheets) and stacks of multiple nano-scaled graphenesheets, which are collectively called nano-scaled graphene plates(NGPs). NGPs could provide unique opportunities for solid statescientists to study the structures and properties of nano carbonmaterials. The structures of these materials may be best visualized bymaking a longitudinal scission on the single-wall or multi-wall of anano-tube along its tube axis direction and then flattening up theresulting sheet or plate. Studies on the structure-property relationshipin isolated NGPs could provide insight into the properties of afullerene structure or nano-tube. Furthermore, these nano materialscould potentially become cost-effective substitutes for carbonnano-tubes or other types of nano-rods for various scientific andengineering applications. The electronic, thermal and mechanicalproperties of NGP materials are expected to be comparable to those ofcarbon nano-tubes; but NGP will be available at much lower costs and inlarger quantities.

Direct synthesis of the NGP material had not been possible, although thematerial had been conceptually conceived and theoretically predicted tobe capable of exhibiting many novel and useful properties. Jang andHuang have provided an indirect synthesis approach for preparing NGPsand related materials [B. Z. Jang and W. C. Huang, “Nano-scaled GraphenePlates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. In most of the priorart methods for making separated graphene platelets, the process beginswith intercalating lamellar graphite flake particles with an expandableintercalation agent (also known as an intercalant or intercalate) toform a graphite intercalation compound (GIC), typically using a chemicaloxidation [e.g., Refes. 1-5, listed below] or an electrochemical (orelectrolytic) method [e.g., Refs. 6, 7, 17-20]. The GIC is characterizedas having intercalate species, such as sulfuric acid and nitric acid,residing in interlayer spaces, also referred to as interstitialgalleries or interstices. In traditional GICs, the intercalant speciesmay form a complete or partial layer in an interlayer space or gallery.If there always exists one graphene layer between two intercalantlayers, the resulting graphite is referred to as a Stage-1 GIC. If ngraphene layers exist between two intercalant layers, we have a Stage-nGIC.) This intercalation step is followed by rapidly exposing the GIC toa high temperature, typically between 800 and 1,100° C., to exfoliatethe graphite flakes, forming vermicular graphite structures known asgraphite worms. Exfoliation is believed to be caused by the interlayervolatile gases, created by the thermal decomposition or phase transitionof the intercalate, which induce high gas pressures inside theinterstices that push apart neighboring graphene layers or basal planes.In some methods, the exfoliated graphite (worms) is then subjected toair milling, air jet milling, ball milling, or ultrasonication forfurther flake separation and size reduction. Conventional intercalationand exfoliation methods and recent attempts to produce exfoliatedproducts or separated platelets are discussed in the followingrepresentative references:

-   1. J. W. Kraus, et al., “Preparation of Vermiculite Paper,” U.S.    Pat. No. 3,434,917 (Mar. 25, 1969).-   2. L. C. Olsen, et al., “Process for Expanding Pyrolytic Graphite,”    U.S. Pat. No. 3,885,007 (May 20, 1975).-   3. A. Hirschvogel, et al., “Method for the Production of    Graphite-Hydrogensulfate,” U.S. Pat. No. 4,091,083 (May 23, 1978).-   4. T. Kondo, et al., “Process for Producing Flexible Graphite    Product,” U.S. Pat. No. 4,244,934 (Jan. 13, 1981).-   5. R. A. Greinke, et al., “Intercalation of Graphite,” U.S. Pat. No.    4,895,713 (Jan. 23, 1990).-   6. F. Kang, “Method of Manufacturing Flexible Graphite,” U.S. Pat.    No. 5,503,717 (Apr. 2, 1996).-   7. F. Kang, “Formic Acid-Graphite Intercalation Compound,” U.S. Pat.    No. 5,698,088 (Dec. 16, 1997).-   8. P. L. Zaleski, et al. “Method for Expanding Lamellar Forms of    Graphite and Resultant Product,” U.S. Pat. No. 6,287,694 (Sep. 11,    2001).-   9. J. J. Mack, et al., “Chemical Manufacture of Nanostructured    Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).-   10. L. M. Viculis and J. J. Mack, et al., “Intercalation and    Exfoliation Routes to Graphite Nanoplatelet,” J. Mater. Chem.,    15 (2005) pp. 974-978.-   11. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for    Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent    Ser. No. 11/509,424 (Aug. 25, 2006).-   12. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Mass Production of    Nano-scaled Platelets and Products,” U.S. patent Ser. No. 11/526,489    (Sep. 26, 2006).-   13. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Method of Producing    Nano-scaled Graphene and Inorganic Platelets and Their    Nanocomposites,” U.S. patent Ser. No. 11/709,274 (Feb. 22, 2007).-   14. Aruna Zhamu, JinJun Shi, Jiusheng Guo, and Bor Z. Jang,    “Low-Temperature Method of Producing Nano-scaled Graphene Platelets    and Their Nanocomposites,” U.S. patent Ser. No. 11/787,442 (Apr. 17,    2007).-   15. Aruna Zhamu, Jinjun Shi, Jiusheng Guo and Bor Z. Jang, “Method    of Producing Exfoliated Graphite, Flexible Graphite, and Nano-Scaled    Graphene Plates,” U.S. patent Ser. No. 11/800,728 (May 8, 2007).-   16. Aruna Zhamu, Joan Jang, Jinjun Shi, and Bor Z. Jang, “Method of    Producing Ultra-thin, Nano-Scaled Graphene Plates,” US Pat.    Application Submitted on Jul. 21, 2007.-   17. N. Watanabe, et al., “Method of Producing a Graphite    Intercalation Compound,” U.S. Pat. No. 4,350,576 (Sep. 21, 1982).-   18. R. A. Greinke, “Expandable Graphite and Method,” U.S. Pat. No.    6,406,612 (Jun. 18, 2002).-   19. R. A. Greinke and R. A. Reynolds, “Expandable Graphite and    Method,” U.S. Pat. No. 6,416,815 (Jun. 18, 2002).-   20. R. A. Greinke, “Intercalated Graphite Flakes Exhibiting Improved    Expansion Characteristics and Process Therefor,” U.S. Pat. No.    6,669,919 (Dec. 30, 2003).

However, these previously invented methods [Refes. 1-10, 17-20] haveseveral serious drawbacks:

-   -   (a) As indicated earlier, in conventional methods, graphite        flakes are intercalated by dispersing the flakes in a solution        containing a mixture of nitric and sulfuric acid. The        intercalation solution may contain other acidic compounds such        as potassium chlorate, chromic acid, potassium permanganate,        potassium chromate, potassium dichromate, perchloric acid, and        the like, or mixtures, such as for example, concentrated nitric        acid and chlorate, chromic acid and phosphoric acid, or mixtures        of a strong organic acid, e.g., trifluoroacetic acid. After the        flakes are intercalated, any excess solution is drained from the        flakes and the flakes are water-washed. The resulting waste        water has to be properly treated (e.g., neutralized) prior to        discharge into the sewage system. Furthermore, the quantity of        intercalation solution retained on the flakes after draining may        range from 20 to 150 parts of solution by weight per 100 parts        by weight of graphite flakes (pph) and more typically about 50        to 120 pph. During the high-temperature exfoliation, the        residual intercalate species retained by the flakes decompose to        produce various species of sulfuric and nitrous compounds (e.g.,        NO_(x) and SO_(x)), which are undesirable. The effluents require        expensive remediation procedures in order not to have an adverse        environmental impact.    -   (b) Typically, exfoliation of the intercalated graphite occurred        at a temperature in the range of 800° C. to 1,100° C. At such a        high temperature, graphite could undergo severe oxidation,        resulting in the formation of graphite oxide, which has much        lower electrical and thermal conductivities compared with        un-oxidized graphite. In recent studies, we have surprisingly        observed that the differences in electrical conductivity between        oxidized and non-oxidized graphite could be as high as several        orders of magnitude.    -   (c) The approach proposed by Mack, et al. [e.g., Refs. 9 and 10]        is a low temperature process. However, Mack's process involves        intercalating graphite with potassium melt, which must be        carefully conducted in a vacuum or an extremely dry glove box        environment since pure alkali metals, such as potassium and        sodium, are extremely sensitive to moisture and pose an        explosion danger. This process is not amenable to mass        production of nano-scaled platelets.    -   (d) Most of the prior art intercalation/exfoliation approaches        were developed for the purpose of producing graphite worms that        are re-compressed to form flexible graphite sheet products. This        purpose is perceived to require maximizing the exfoliation        volume of a graphite sample. Non-judicious practice of        maximizing the expansion volume often occurred at the expense of        reduced uniformity in exfoliation, i.e., certain portion of a        graphite particle being exfoliated to a great extent, but other        portions remaining intact. Graphite worms of this nature are not        the most suitable for the production of separated, nano-scaled        graphene platelets.    -   (e) Although prior art intercalation-exfoliation methods might        be capable of sporadically producing a small amount of        ultra-thin graphene platelets (e.g., 1-5 layers), most of the        platelets produced are much thicker than 10 nm. Many of the NGP        applications require the NGPs to be as thin as possible; e.g.,        as a supercapacitor electrode material. Hence, it is desirable        to have a method that is capable of consistently producing        ultra-thin NGPs.

Hence, an urgent need exists to have environmentally benign intercalatesthat do not lead to the effluent of undesirable chemical species intothe drainage (e.g., sulfuric acid) or into the air (e.g., SO₂ and NO₂).It is further desirable to have a graphite intercalation compound thatdoes not require a high exfoliation temperature. It is also desirable tohave a GIC that can be more uniformly exfoliated for the production ofnano-scaled graphene platelets that are more uniform in sizes. It ishighly desirable to have a method of expanding a laminar (layered)compound or element, such as graphite and graphite oxide (partiallyoxidized graphite), to produce ultra-thin graphite and graphite oxideflakes or platelets, with an average thickness smaller than 2 nm orthinner than 5 layers. Most preferably, the method of intercalation andexfoliation should be applicable to not only natural flake graphite, butalso synthetic graphite, highly oriented pyrolytic graphite, graphiteoxide, graphite fiber, graphitic nano-fiber, and the like.

In order to meet these goals, we investigated potentially viableintercalates that contain no undesirable elements, such as N, S, P, As,Se, transition metal, or halogen element. In particular, we focused ourstudies on chemical species that contain only H, C, and O atoms, whichare expected to produce no contaminants. We have found that carboxylicacids, such as formic, acetic, propionic, butyric, pentanoic, andhexanoic acids and their anhydrides, are particularly suitable formeeting our objectives.

It may be noted that Kang, et al [Ref. 7] used an electrochemical methodto intercalate natural flake graphite with formic acid for the purposeof producing flexible graphite products. However, there was noindication that the formic acid-intercalated graphite could lead towell-separated, nano-scaled graphene platelets (NGPs), let alone NGPs ofuniform sizes or ultra-thinness (e.g., thinner than 2 nm). Furthermore,there was no indication, implicit or explicit, that any other member ofthe carboxylic acid series or any member of their anhydrides and theirderivatives could be successfully intercalated into interstices ofgraphite. There was also no indication that any of the carboxylic acidcan be intercalated into other layered graphite structures (e.g.,graphite fibers) than natural flake graphite. Further, although Greinke,et al [Ref. 18-20] used carboxylic acids as an “expansion aid” in theformation of expandable graphite (GICs) for the purpose of producingflexible graphite products, the intercalate in these GICs was sulfuricacid. The method comprises “contacting graphite flake with an organicexpansion aid” [Claim 1 of Ref. 18]. It is speculated that the organicexpansion aid (e.g., carboxylic acid), under the experimental conditionsof Greinke, et al., resided on the exterior surface of graphiteparticles or between graphite particles. There was no indication thatcarboxylic acid, when used alone or as a portion of an intercalationsolution in [Refs. 18-20], could penetrate and stay in graphiteinterstices to form a stable GIC. The organic expansion aid was used toincrease the macroscopic expansion volume of a graphite sample, not forimproving uniform expansion of individual flakes in a graphite particle,nor for enhancing the separation of exfoliated flakes. There was noattempt to apply this approach to exfoliation of other graphitestructures than natural flake graphite. There was no attempt onsubmitting the exfoliated graphite to a mechanical shearing treatmentfor the purpose of producing NGPs.

By contrast, after intensive studies, we have observed that asignificant amount of a wide range of carboxylic acids, not just formicacid, can be electrochemically intercalated into graphite to form stablegraphite intercalation compounds (GICs). These GICs, when exposed to atemperature in the range of 300-800° C. (preferably in the range of400-600° C.), were exfoliated in a relatively uniform manner. Theresulting exfoliated flakes can be readily separated, via a mechanicalshearing treatment, into individual nano-scaled graphene platelets(NGPs) that are relatively uniform in thickness. This was achievedwithout using an undesirable acid like sulfuric acid or undesirableoxidizing agent like nitric acid or potassium permanganate.

SUMMARY OF THE INVENTION

The present invention provides a method of producing nano-scaledgraphene platelets with an average thickness smaller than 30 nm from alayered graphite material. The method comprises (a) forming a carboxylicacid-intercalated graphite compound by an electrochemical reaction whichuses a carboxylic acid as both an electrolyte and an intercalate source,the layered graphite material as an anode material, and a metal orgraphite as a cathode material, and wherein a current is imposed uponthe cathode and the anode at a current density for a duration of timesufficient for effecting the electrochemical reaction; (b) exposing theintercalated graphite compound to a thermal shock to produce exfoliatedgraphite; and (c) subjecting the exfoliated graphite to a mechanicalshearing treatment to produce the nano-scaled graphene platelets. Theexfoliation step in the instant invention does not involve the evolutionof undesirable species, such as NO_(x), and SO_(x), which are commonby-products of exfoliating conventional sulfuric or nitricacid-intercalated graphite compounds.

The carboxylic acid may be selected from the group consisting ofaromatic carboxylic acid, aliphatic or cycloaliphatic carboxylic acid,straight chain or branched chain carboxylic acid, saturated andunsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylicacids that have 1-10 carbon atoms, alkyl esters thereof, andcombinations thereof. Preferably, the carboxylic acid is selected fromthe group consisting of saturated aliphatic carboxylic acids of theformula H(CH₂)_(n)COOH, wherein n is a number of from 0 to 5, includingformic, acetic, propionic, butyric, pentanoic, and hexanoic acids,anhydrides thereof, reactive carboxylic acid derivatives thereof, andcombinations thereof. The most preferred carboxylic acids are formicacid and acetic acid.

The layered graphite material may be selected from natural graphite,synthetic graphite, highly oriented pyrolytic graphite, graphite fiber,graphitic nano-fiber, graphite oxide, graphite fluoride, chemicallymodified graphite, or a combination thereof. The mechanical shearingtreatment preferably comprises using air milling (including air jetmilling), ball milling, mechanical shearing (including rotating bladefluid grinding), ultrasonication, or a combination thereof.

The imposing current used in the electrochemical reaction preferablyprovides a current density in the range of 20 to 600 A/m², morepreferably in the range of 50 to 400 A/m², and most preferably in therange of 100 to 300 A/m². The exfoliation step preferably comprisesheating the intercalated graphite to a temperature in the range of300-800° C. for a duration of 10 seconds to 2 minutes, most preferablyat a temperature in the range of 400-600° C. for a duration of 30-60seconds.

Depending upon the nature of intercalate and the type of laminargraphite, the resulting NGPs, after one intercalation and exfoliationcycle, exhibit an average platelet thickness in the range of 10-30 nm.For a given graphite type and a carboxylic acid, the resulting NGPs havea relatively narrow distribution of thicknesses, as compared with NGPsprepared from conventional sulfuric acid-intercalated graphiteexfoliated at a comparable temperature.

The method may comprise additional steps of electrochemicalintercalation of the exfoliated graphite or subsequently separatednano-scaled graphene platelets to obtain a further intercalatedcompound, and exfoliation of the further intercalated compound toproduce thinner nano-scaled graphene platelets, with or without anothermechanical shearing treatment. Repeated intercalations and exfoliationsresult in the formation of ultra-thin NGPs with an average thickness nogreater than 2 nm or 5 layers.

It may be noted that Viculis, et al [Ref. 10] did report that “graphitenanoplatelets with thickness down to 2-10 nm are synthesized by alkalimetal intercalation followed by ethanol exfoliation and microwavedrying.” This was achieved by intercalating graphite with an oxidizingacid to form a graphite intercalation compound (GIC), exfoliating theGIC, re-intercalating the exfoliated graphite with an alkali metal toform a first-stage (Stage-1) compound, reacting the first-stage compoundwith ethanol to exfoliate the compound, and further separating theexfoliated graphite with microwave heating. That alkali metals reactviolently with water and alcohol implies that Visculis's method can notbe a safe and reliable process for mass-producing NGPs. Furthermore,although re-intercalation and re-exfoliation were used in this processand first-stage graphite compound was obtained, the resulting graphiteplatelets are no thinner than 2 nm. Most of the platelets are thickerthan 10-15 nm even after further exfoliation and separation viamicrowave heating (e.g., FIG. 5 of Ref. 10). Re-intercalation by liquideutectic of sodium and potassium (NaK₂) and subsequent exfoliationyielded platelets with thicknesses of 2-150 nm (Page 976 of Ref. 10). Itseems that violent reactions between intercalated alkali metals andwater or ethanol tend to result in highly non-uniform exfoliation. Evenwith alkali metal-intercalated graphite being mostly Stage-1, theresulting platelets exhibit such a wide range of thicknesses (2-150 nm).By contrast, our invented method consistently produces platelets with anaverage thickness thinner than 2 nm or 5 layers after repeatingelectrochemical intercalation and exfoliation by only one cycle.

Although one of our co-pending applications [Ref. 16] provides a methodthat involves repeated intercalations and exfoliations, theintercalation was based on chemical oxidation (e.g., sulfuric acid andan oxidizing agent), rather than electrochemical intercalation.Furthermore, in the present invention, if the NGPs or exfoliated flakeshave an average thickness smaller than 30 nm (as opposed to 10 nm in theco-pending application case), the electrochemically re-intercalatedcompound consistently led to the formation of ultra-thin NGPs. This is asurprising result beyond and above what is anticipated by the co-pendingapplication [Ref. 16].

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A flow chart showing a two-stage process of producing ultra-thingraphite platelets (NGPs with an average thickness thinner than 2 nm or5 layers). Both steps entail an electrochemical intercalation step.

FIG. 2 Schematic of an apparatus for electrochemical intercalation ofgraphite.

FIG. 3 Transmission electron micrographs of NGPs: (A) NGP with anaverage thickness <30 nm (at the end of Phase I); (B) ultra-thin NGPs(Phase II).

FIG. 4 Platelet thickness distributions for NGPs prepared from prior artsulfuric-nitric acid-intercalated graphite and formic-acid intercalatedgraphite.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Carbon materials can assume an essentially amorphous structure (glassycarbon), a highly organized crystal (graphite), or a whole range ofintermediate structures that are characterized in that variousproportions and sizes of graphite crystallites and defects are dispersedin an amorphous matrix. Typically, a graphite crystallite is composed ofa number of graphene sheets or basal planes that are bonded togetherthrough van der Waals forces in the c-axis direction, the directionperpendicular to the basal plane. These graphite crystallites aretypically micron- or nanometer-sized. The graphite crystallites aredispersed in or connected by crystal defects or an amorphous phase in agraphite particle, which can be a graphite flake, carbon/graphite fibersegment, carbon/graphite whisker, or carbon/graphite nano-fiber. In thecase of a carbon or graphite fiber segment, the graphene plates may be apart of a characteristic “turbostratic structure.”

One preferred specific embodiment of the present invention is a methodof producing a nano-scaled graphene plate (NGP) material that isessentially composed of a sheet of graphene plane or multiple sheets ofgraphene plane stacked and bonded together (on an average, up to fivesheets per plate). Each graphene plane, also referred to as a graphenesheet or basal plane, comprises a two-dimensional hexagonal structure ofcarbon atoms. Each plate has a length and a width parallel to thegraphite plane and a thickness orthogonal to the graphite plane. Bydefinition, the thickness of an NGP is 100 nanometers (nm) or smaller,with a single-sheet NGP being as thin as 0.34 nm. The length and widthof a NGP are typically between 1 μm and 20 μm, but could be longer orshorter. For certain applications, both length and width are smallerthan 1 μm. In addition to graphite, graphite oxide and graphite fluorideare another two of the many examples of laminar or layered materialsthat can be exfoliated to become nano-scaled platelets.

Generally speaking, a method has been developed for converting a layeredor laminar graphite material 10 to nano-scaled graphite platelets havingan average thickness smaller than 30 nm, most often smaller than 10 nm.As schematically shown in the upper portion of FIG. 1, the methodcomprises (a) forming a carboxylic acid-intercalated graphite compound12 by an electrochemical reaction which uses a carboxylic acid as bothan electrolyte and an intercalate source, the layered graphite materialas an anode material, and a metal or graphite as a cathode material, andwherein a current is imposed upon the cathode and the anode at a currentdensity for a duration of time sufficient for effecting theelectrochemical reaction; (b) exposing the intercalated graphitecompound 12 to a thermal shock to produce exfoliated graphite 14; and(c) subjecting the exfoliated graphite to a mechanical shearingtreatment to produce the desired nano-scaled graphene platelets 16. Theexfoliation step preferably comprises heating the intercalated graphiteto a temperature in the range of 300-800° C. for a duration of 10seconds to 2 minutes, most preferably at a temperature in the range of400-600° C. for a duration of 30-60 seconds. The exfoliation step in theinstant invention does not involve the evolution of undesirable species,such as NO_(x) and SO_(x), which are common by-products of exfoliatingconventional sulfuric or nitric acid-intercalated graphite compounds.

Schematically shown in FIG. 2 is an apparatus that can be used forelectrochemical intercalation of graphite according to a preferredembodiment of the present invention. The apparatus comprises a container32 to accommodate electrodes and electrolyte. The anode is comprised ofmultiple graphite particles 40 that are dispersed in an electrolyte(e.g., a carboxylic acid, which is also an intercalate) and aresupported by a porous anode supporting element 34, preferably a porousmetal plate, such as platinum or lead. The graphite particles 40preferably form a continuous electron path with respect to the anodesupport plate 34, but are accessible to the intercalate. An electricallyinsulating, porous separator plate 38 (e.g., Teflon fabric or glassfiber mat) is placed between the anode and the cathode 36 (e.g., aporous graphite or metal plate) to prevent internal short-circuiting. ADC current source 46 is used to provide a current to the anode supportelement 34 and the cathode 36. The imposing current used in theelectrochemical reaction preferably provides a current density in therange of 50 to 600 A/m², most preferably in the range of 100 to 400A/m². Fresh electrolyte (intercalate) may be supplied from anelectrolyte source (not shown) through a pipe 48 and a control valve 50.Excess electrolyte may be drained through a valve 52.

The carboxylic acid, containing only C, H, and O atoms, may be selectedfrom the group consisting of aromatic carboxylic acid, aliphatic orcycloaliphatic carboxylic acid, straight chain or branched chaincarboxylic acid, saturated and unsaturated monocarboxylic acids,dicarboxylic acids and polycarboxylic acids that have 1-10 carbon atoms,alkyl esters thereof, and combinations thereof. Preferably, thecarboxylic acid is selected from the group consisting of saturatedaliphatic carboxylic acids of the formula H(CH₂)_(n)COOH, wherein n is anumber of from 0 to 5, including formic, acetic, propionic, butyric,pentanoic, and hexanoic acids, anhydrides thereof, reactive carboxylicacid derivatives thereof, and combinations thereof. In place of thecarboxylic acids, the anhydrides or reactive carboxylic acid derivativessuch as alkyl esters can also be employed. Representative of alkylesters are methyl formate and ethyl formate. The most preferredcarboxylic acids are formic acid and acetic acid.

Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Among the polycarboxylic acids, citric acidis preferred due to its availability and low cost.

The carboxylic acid-intercalated graphite can be easily exfoliated byrapidly heating the GIC to a desired exfoliation temperature. Anadvantage of such a GIC in comparison with the prior art GICs is thatonly H, C and O are released into the atmosphere during the exfoliationprocess. Depending on the applied current density and the reaction time,an expansion volume of from 100-300 ml/g, at 400-800° C., and volatilecontent of 10-20 wt %, could be obtained. The residual sulfur content inthe expanded graphite is no more than the sulfur impurity level of theoriginal graphite flakes since the process is totally sulfur free, asopposed to more than 1,000 ppm of sulfur typically found in conventionalexfoliated graphite manufactured from a sulfuric acid-intercalated GIC.Furthermore, the exfoliated graphite and subsequent NGPs do not containadditional corrosive species such as chlorine, fluorine, nitrogen, andphosphor.

The layered graphite material may be selected from natural graphite,synthetic graphite, highly oriented pyrolytic graphite, graphite fiber,graphitic nano-fiber, graphite oxide, graphite fluoride, chemicallymodified graphite, graphite intercalation compound, exfoliated graphite,or a combination thereof.

The mechanical shearing treatment, used to further separate graphiteflakes and possibly reduce the flake size, preferably comprises usingair milling (including air jet milling), ball milling, mechanicalshearing (including rotating blade fluid grinding), any fluid energybased size-reduction process, ultrasonication, or a combination thereof.

We have found that the invented electrochemical intercalation andthermal exfoliation mostly led to the formation of NGPs with an averagethickness smaller than 10 nm. However, intercalation with higher-orderaliphatic carboxylic acids of the formula H(CH₂)_(n)COOH with n greaterthan 2 (i.e., butyric, pentanoic, and hexanoic acids) or withdicarboxylic acids and polycarboxylic acids could lead to the formationof NGPs with an average thickness greater than 10 nm, but smaller than30 nm. In order to further reduce the platelet thickness, we haveconducted further studies and found that repeated electrochemicalintercalations/exfoliations are an effective method of producingultra-thin, nano-scaled graphene platelets with an average thicknesssmaller than 2 nm or 5 graphene sheets.

Hence, another preferred embodiment is a method of producing ultra-thinNGPs with the method comprising: (a) forming a carboxylicacid-intercalated graphite compound by an electrochemical reaction whichuses a carboxylic acid as both an electrolyte and an intercalate source,said layered graphite material as an anode material, and a metal orgraphite as a cathode material, and wherein a current is imposed uponthe cathode and the anode at a current density for a duration of timesufficient for effecting the electrochemical reaction; (b) exposing theintercalated graphite compound to a thermal shock to produce exfoliatedgraphite; (c) re-intercalating the exfoliated graphite by repeating step(a) using the exfoliated graphite (with or without a mechanical shearingtreatment) as an anode material to produce a further-intercalatedgraphite compound; and (d) exposing the further intercalated graphitecompound to a thermal shock to produce the desired ultra-thin,nano-scaled graphene platelets. Step (d) may further comprise a sub-stepof subjecting the thermal shock exposed graphite to a mechanicalshearing treatment to further separate or size-reduce the platelets.

In summary, the method may be described as having two primary phases,schematically shown in FIG. 1. Phase I essentially entails converting alaminar graphite material to exfoliated graphite flakes 14 or NGPs 16 ofintermediate thicknesses (typically, on an average, thinner than 30 nm;e.g., FIG. 3(A)). In one preferred embodiment of the present invention,Phase II entails essentially repeating Phase I to further reduce theaverage platelet thickness by re-intercalating the exfoliated flakes 14or NGPs 16 to obtain re-intercalated compound 20, exfoliating there-intercalated compound to either directly produce ultra-thin NGPs 24or produce further exfoliated NGPs 22, which are then subjected to amechanical shearing treatment to obtain ultra-thin NGPs 24 (e.g., FIG.3(B)).

After extensive and in-depth studies on the preparation of NGPs, we havesurprisingly observed that the NGPs, after first cycle ofelectro-chemical intercalation and thermal exfoliation, exhibit anaverage thickness thinner than 30 nm (thinner than 10 nm if formic acidor acetic acid was used). These NGPs of intermediate thicknesses (e.g.,thinner than 30 nm), upon repeated electrochemical intercalation andthermal exfoliation for another cycle, led to the formation ofultra-thin NGPs. The re-intercalation/exfoliation procedures orconditions used in Phase II are fundamentally no different than thoseused in Phase I.

It may be noted that, in a traditional GIC obtained by intercalation ofa laminar graphite material, the intercalant species may form a completeor partial layer in an inter-layer space or gallery. If there alwaysexists one graphene layer between two intercalant layers, the resultinggraphite is referred to as a Stage-1 GIC. If n graphene layers existbetween two intercalant layers, we have a Stage-n GIC. Carboxylicacid-intercalated graphite compounds were found to be stage-2, stage-3,stage-4, or stage-5, depending on the type of carboxylic acid used. Itis generally believed that a necessary condition for the formation ofall single-sheet NGPs is to have a perfect Stage-1 GIC for exfoliation.Even with a Stage-1 GIC, not all of the graphene layers get exfoliatedfor reasons that remain unclear. Similarly, exfoliation of a Stage-n GIC(with n>5) tends to lead to a wide distribution of NGP thicknesses(mostly much greater than n layers). In other words, exfoliation ofStage-5 GICs often yields NGPs much thicker than 10 or 20 layers. Hence,a major challenge is to be able to consistently produce NGPs withwell-controlled dimensions (preferably ultra-thin) fromacid-intercalated graphite.

In this context, it was surprising for us to discover that, once thestarting NGPs are thinner than 10 nm, re-intercalation by chemicaloxidation tends to lead to mostly Stage-1 and Stage-2 GICs, as indicatedin a co-pending application [Ref. 16]. It is further surprising thatNGPs or flakes with an average thickness thinner than 30 nm, uponelectrochemical intercalation with a carboxylic acid and thermalexfoliation for another cycle, become ultra-thin NGPs. Many of theseNGPs are single-sheet or double-sheet NGPs, with very few NGPs thickerthan 5 layers. Thus, one can conclude that re-intercalation/exfoliationis an effective, consistent way of producing ultra-thin NGPs with anaverage thickness less than 2 nm (or 5 layers), usually less than 1 nm.We have further observed that repeated intercalations and exfoliationscan be performed to obtain mostly single-sheet NGPs. Now, one canconsistently produce single-sheet NGPs for a wide range of industrialuses.

The following examples serve to provide the best modes of practice forthe present invention and should not be construed as limiting the scopeof the invention:

EXAMPLE 1 Nano-Scaled Graphene Platelets (NGPs) From Highly OrientedPyrolytic Graphite (HOPG) Flakes Via Repeated ElectrochemicalIntercalation and Exfoliation Steps

One gram of HOPG flakes, ground to approximately 20 μm or less in sizes,were used as the anode material and 1,000 mL of formic acid was used asthe electrolyte and intercalate source. The anode supporting element isa platinum plate and the cathode is a graphite plate of approximately 4cm in diameter and 0.2 cm in thickness. The separator, a glass fiberfabric, was used to separate the cathode plate from the graphite flakesand to compress the graphite flakes down against the anode supportingelement to ensure that the graphite flakes are in electrical connectionwith the anode supporting element to serve as the anode. The electrodes,electrolyte, and separator are contained in a Buchner-type funnel toform an electrochemical cell. The anode supporting element, the cathode,and the separator are porous to permit intercalate (electrolyte) tosaturate the graphite and to pass through the cell from top to bottom.

The graphite flakes were subjected to an electrolytic oxidationtreatment at a current of 0.5 amps (current density of about 0.04amps/cm²) and at a cell voltage of about 4-6 volts for 2-5 hours. Thesevalues may be varied with changes in cell configuration and makeup.Following electrolytic treatment, the resulting intercalated flake waswashed with water and dried.

Subsequently, approximately ⅔ of the intercalated compound wastransferred to a furnace pre-set at a temperature of 600° C. for 30seconds. The compound was found to induce extremely rapid and highexpansions of graphite crystallites with an expansion ratio of greaterthan 200. The thickness of individual platelets ranged from two graphenesheets to approximately 43 graphene sheets (average of 23 sheets orapproximately 7.9 nm) based on SEM and TEM observations.

Approximately one half of these NGPs were then subjected tore-intercalation under comparable electrolytic oxidation conditions toobtain re-intercalated NGPs. Subsequently, these re-intercalated NGPswere transferred to a furnace pre-set at a temperature of 600° C. for 30seconds to produce ultra-thin NGPs. Electron microscopic examinations ofselected samples indicate that the majority of the resulting NGPscontain between single graphene sheet and five sheets.

COMPARATIVE EXAMPLE 1 Sulfuric-Nitric Acid-Intercalated HOPG

One gram of HOPG flakes as used in Example 1 were intercalated with amixture of sulfuric acid, nitric acid, and potassium permanganate at aweight ratio of 4:1:0.05 (graphite-to-intercalate ratio of 1:3) for fourhours. Upon completion of the intercalation reaction, the mixture waspoured into deionized water and filtered. The sample was then washedwith 5% HCl solution to remove most of the sulfate ions and residualsalt and then repeatedly rinsed with deionized water until the pH of thefiltrate was approximately 5. The dried sample was then exfoliated at800° C. for 45 seconds. A sample of formic acid-intercalated graphiteprepared in Example 1 was also exfoliated at 800° C. for 45 seconds.Both samples were separately submitted to a mechanical shearingtreatment using a Cowles rotating blade device for 30 minutes. Theresulting NGPs were examined using SEM and TEM and their thicknessdistributions are shown in FIG. 4. It is clear that the formicacid-intercalated graphite led to a narrower thickness distribution(more uniform size) and a smaller average thickness.

EXAMPLE 2 NGPs from Natural Graphite Flakes Intercalated with AceticAcid

One gram of natural flake graphite having 50 mesh particle size wassubjected to the same electrochemical intercalation conditions describedin Example 1, with formic acid replaced by acetic acid. The graphiteflakes were subjected to an electrolytic oxidation treatment at acurrent of 0.5 amps (current density of about 0.04 amps/cm²) and at acell voltage of about 6 volts for 3 hours. Following the electrolytictreatment, the resulting intercalated flake was washed with water anddried. Subsequently, the intercalated compound was transferred to afurnace pre-set at a temperature of 500° C. for 45 seconds. The compoundwas found to induce extremely rapid and high expansions of graphitecrystallites with an expansion ratio of greater than 100. After amechanical shearing treatment in a laboratory-scale Cowles rotatingblade device for 15 minutes, the resulting NGPs exhibit a thicknessranging from three graphene sheets to approximately 50 graphene sheets(average of 25 sheets or approximately 8.5 nm) based on SEM and TEMobservations. Re-intercalation of these NGPs and subsequent exfoliationresulted in further reduction in platelet thickness, with an averagethickness of approximately 1.4 nm.

EXAMPLE 3 Repeated Interaction, Exfoliation, and Separation Steps ofGraphite Oxide

Graphite oxide was prepared by oxidation of graphite flakes withsulfuric acid, nitrate, and potassium permanganate according to themethod of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The sample was then washed with 5% HCl solution to removemost of the sulfate ions and residual salt and then repeatedly rinsedwith deionized water until the pH of the filtrate was approximately 7.The intent was to remove all sulfuric and nitric acid residue out ofgraphite interstices. The slurry was spray-dried and stored in a vacuumoven at 60° C. for 24 hours. The interlayer spacing of the resultinglaminar graphite oxide was determined by the Debey-Scherrer X-raytechnique to be approximately 0.73 nm (7.3 Å), indicating that graphitehas been converted into graphite oxide.

The dried intercalated graphite oxide powder sample was then subjectedto an electrochemical intercalation under comparable conditions asdescribed in Example 1, but with formic acid replaced by propionic acidas the intercalate. The dried, intercalated compound was placed in aquartz tube and inserted into a horizontal tube furnace pre-set at 650°C. for 35 seconds. The exfoliated worms were mixed with water and thensubjected to a mechanical shearing treatment using a Cowlesrotating-blade shearing machine for 20 minutes. The resulting flakes(intermediate-thickness platelets) were found to have a thickness of12.6 nm.

These intermediate-thickness platelets were then re-intercalatedelectrochemically under comparable conditions. The dried powder samplewas placed in a quartz tube and inserted into a horizontal tube furnacepre-set at 800° C. for 35 seconds. The resulting ultra-thin graphiteoxide platelets have an average thickness of approximately 1.8 nm.

EXAMPLE 4 NGPs from Short Carbon Fiber Segments

The procedure was similar to that used in Example 1, but the startingmaterial was graphite fibers chopped into segments with 0.2 mm orsmaller in length prior to electrochemical intercalation by citric acid.The diameter of carbon fibers was approximately 12 μm. Afterintercalation and exfoliation at 600° C. for 30 seconds, the plateletsexhibit an average thickness of 18 nm. Electrochemical re-intercalationof these intermediate-thickness NGPs with formic acid and subsequentexfoliation of the dried re-intercalation compound resulted in theformation of ultra-thin NGPs with an average thickness of 1.7 nm.

EXAMPLE 5 NGPs from Carbon Nano-Fibers (CNFs)

A powder sample of carbon nano-fibers was supplied from Applied Science,Inc. (ASI), Cedarville, Ohio. Approximately 1 gram of CNFs was subjectedto repeated electrochemical intercalations and exfoliations underconditions similar to those used in Example 1. Ultra-thin NGPs with anaverage thickness of 1.5 nm were obtained.

It is important to emphasize again the notion that, although Kang, et al[Ref. 7] used an electrochemical method to intercalate natural flakegraphite with formic acid, there was no indication, implicit orexplicit, that any other member of the carboxylic acid family or anymember of their anhydrides and their derivatives could be successfullyintercalated into interstices of graphite. This is not a trivial orobvious matter due to the fact that formic acid with a chemical formulaHCOOH is the smallest molecule in the family of carboxylic acids. Othermembers are much bigger molecules. Yet, the interstitial spaces betweentwo basal planes are only approximately 0.28 nm (the plane-to-planedistance is 0.34 nm). A skilled person in the art would predict thatlarger carboxylic acid molecules can not intercalate into interstitialspaces of a layered graphite material. After intensive R&D efforts, wefound that electrochemical methods with a proper combination of anintercalate and an imposing current density could be used to open up theinterstitial spaces to accommodate much larger carboxylic acid moleculesthan formic acid.

Furthermore, Kang, et al. did not indicate that any of the carboxylicacid, formic acid or larger-molecule acids, can be intercalated intoother layered graphite structures (such as graphite fibers, carbonnano-fibers, synthetic graphite, or highly pyrolytic graphite flakes)than natural flake graphite. There was no indication that the formicacid-intercalated graphite could lead to well-separated, nano-scaledgraphene platelets (NGPs), let alone NGPs of uniform sizes orultra-thinness (e.g., thinner than 2 nm).

The invention claimed is:
 1. A method of producing ultra-thin,nano-scaled graphene platelets with an average thickness smaller than 2nm or 5 graphene sheets from a layered graphite material, said methodcomprising: a) forming a carboxylic acid-intercalated graphite compoundby an electrochemical reaction which uses a carboxylic acid orderivative as both an electrolyte and an intercalate source, saidlayered graphite material as an anode material, and a metal or graphiteas a cathode material, and wherein a current is imposed upon saidcathode and said anode at a current density for a duration of timesufficient for effecting said electrochemical reaction; b) rapidlyheating said carboxylic acid-intercalated graphite compound to a desiredtemperature to produce exfoliated graphite; c) re-intercalating saidexfoliated graphite by repeating step (a) using said exfoliated graphiteas an anode material to form a further carboxylic acid-intercalatedgraphite compound; d) rapidly heating said furthercarboxylic-intercalated graphite compound; e) subjecting a graphitecompound obtained in step (d) to a mechanical shearing treatment toproduce said ultra-thin, nano-scaled graphene platelets with an averagethickness smaller than 2 nm or 5 graphene sheets; and f) recovering saidgraphene platelets with an average thickness smaller than 2 nm or 5graphene sheets.
 2. The method of claim 1 wherein said layered graphitematerial is selected from natural graphite, synthetic graphite, highlyoriented pyrolytic graphite, graphite fiber, graphitic nano-fiber,graphite oxide, graphite fluoride, chemically modified graphite, or acombination thereof.
 3. The method of claim 1 wherein said carboxylicacid or derivative is selected from the group consisting of aromaticcarboxylic acid, aliphatic or cycloaliphatic carboxylic acid, straightchain or branched chain carboxylic acid, saturated and unsaturatedmonocarboxylic acids, dicarboxylic acids, polycarboxylic acids that have1-10 carbon atoms, alkyl esters thereof, and combinations thereof. 4.The method of claim 1 wherein said carboxylic acid or derivative isselected from saturated aliphatic carboxylic acids of the formulaH(CH₂)_(n)COOH, wherein n is a number of from 0 to 5, anhydridesthereof, reactive carboxylic acid derivatives thereof, and a combinationthereof.
 5. The method of claim 1 wherein said mechanical shearingtreatment comprises using air milling, air jet milling, ball milling,rotating-blade mechanical shearing, ultrasonication, or a combinationthereof.
 6. The method of claim 1 wherein the imposing current providesa current density in the range of 50 to 600 A/m².
 7. The method of claim1 wherein the imposing current provides a current density in the rangeof 100 to 400 A/m².
 8. The method of claim 1 wherein said step (b) or(d) comprises heating said carboxylic acid-intercalated graphitecompound or said further carboxylic acid-intercalated graphite compoundto a temperature in the range of 300-800° C. for a duration of 15seconds to 2 minutes.
 9. The method of claim 1 wherein said ultra-thin,nano-scaled graphene platelets comprise single graphene-sheet and/ordouble graphene-sheet platelets.